Conventionally, there have been disclosed various types of wireless transceivers employing a superheterodyne scheme that converts a radio frequency signal into a relatively low frequency signal (intermediate frequency signal), and amplifying and detecting the same. For example, a wireless receiver disclosed in Patent Document 1 includes a local oscillator which outputs a signal (i.e., a local oscillation signal) of a frequency (local oscillation frequency) that is an integer multiple of the frequency of an input signal (reference oscillation signal).
In the wireless receiver, a reception signal (RF signal) received by an antenna and the local oscillation signal outputted from the local oscillator are mixed by a mixer and converted into a signal having a frequency (intermediate frequency) lower than that of the RF signal. Further, various wireless transceivers using a phase locked loop (PLL) circuit as a local oscillator have also been provided.
As for a wireless communications station, there may be cases where characteristics (RF characteristics) of radio waves in use such as an occupied frequency bandwidth, an adjacent channel leakage power or the like should meet the rules of Radio Regulation Law. For example, in Japan Radio Regulation Law, a different standard (communication standards) is prescribed for each of usage purposes. In particular, a ‘low power radio station’ is prescribed as one of radio stations not requiring license in a provisory clause, article 4 of Japan Radio Regulation Law.
The ‘low power radio station’ includes a ‘radio station for a cordless phone’, a ‘particular low power radio station’, a ‘low power security system’, a ‘radio station for a low power data communication system’ and the like. Standards for radio facilities of each radio station are prescribed by facility regulation of enforcement regulations of the same Law.
As a wireless communication system including a particular low power radio station, for example, a fire warning system disclosed in Patent Document 2 was already proposed. This fire warning system includes multiple fire alarms as radio stations installed in multiple locations.
Each of the fire alarms includes a fire detection unit for detecting a fire, an alarm unit for generating an alarm sound, a wireless transmission/reception unit for transmitting and receiving fire notification information notifying about the occurrence of a fire through radio signals, and an operation controller (or a microcomputer) for controlling operations of the alarm unit and the wireless transmission/reception unit.
When a fire detection unit of a fire alarm detects the occurrence of a fire, an operation controller of the fire alarm outputs an alarm sound from an alarm unit and, simultaneously, a wireless transmission/reception unit thereof transmits fire notification information to other fire alarms. When wireless transmission/reception units of the other fire alarms receive the fire notification information from the fire alarm at the origin of fire, alarm units of the other fire alarms make an alarm sound loudly. Thus, when a fire alarm at a certain location detects the occurrence of a fire, alarm sounds are outputted from all multiple fire alarms including the fire alarm at the origin of fire, thereby quickly and reliably notifying about the occurrence of fire.
As the above, the fire alarm transmits the fire notification information through a radio signal and uses a battery as a power source. This eliminates a necessity of wiring and increases freedom of installation position. However, since a fire alarm is usually installed at a high location (e.g., the ceiling) where it is not easy to access in maintenance (e.g., battery replacement), preferably, the fire alarm can be used for a long period of time, e.g., for years, without maintenance and power consumption thereof is reduced to thereby lengthen a life span of the battery.
To this end, in each fire alarm, the operation controller including a microcomputer is switched into a sleep state consuming less power and a transmission/reception operation of the wireless transmission/reception unit is stopped, except for a case of detecting a fire and sounding an alarm, and wirelessly transmitting fire notification information. However, when the operation controller is in the sleep state except for the case of the fire detection, fire notification information wirelessly transmitted from another fire alarm cannot be received. For that reason, each fire alarm intermittently starts the operation controller in the sleep state to execute an operation of receiving a wireless signal.
Specifically, when a start signal is inputted to the operation controller from a timer, the operation controller checks whether or not it can receive a radio wave (i.e., fire notification information wirelessly transmitted from another fire alarm). That is, the operation controller controls the wireless transmission/reception unit to perform a receiving operation, and determines whether or not the strength of a reception signal received by the wireless transmission/reception unit exceeds a certain reference value.
When the reception signal strength does not exceed the reference value, the operation controller stops the transmission and reception operation of the wireless transmission/reception unit, sets an intermittent reception time period in the timer for next intermittent reception, starts counting, and transitions to the sleep state. On the other hand, when the reception signal strength exceeds the reference value, the operation controller continues the reception state of the wireless transmission/reception unit, analyzes the reception signal received by the wireless transmission/reception unit, and determines whether or not there is any communications to the fire alarm itself. When there is any communications to the fire alarm, the operation controller of the fire alarm executes corresponding processing.
Thus, the operation controller operates intermittently, and checks the signal strength of a radio wave received by the wireless transmission/reception unit. When the operation controller determines that the radio wave cannot be received, the operation controller stops the transmission and reception operation of the wireless transmission/reception unit, thereby reducing power consumption and lengthening a life span of a battery.
Meanwhile, a conventional wireless communication device is disclosed in Patent Document 3. As shown in FIG. 15, the wireless communication device includes an antenna 1000, an RF unit 1100, an interface unit 1200, and a microcomputer unit 1300. The RF unit 1100 includes a demodulation unit 111 for demodulating reception data (a demodulation signal) from a radio signal received via the antenna 1000 and a sampling clock generating unit 112 for generating a sampling clock from a synchronization bit stream of the demodulation signal.
The interface unit 1200 includes a frame code register 121 for storing a frame synchronization part (a unique word), a frame synchronization shift register 122 for sequentially storing the reception data demodulated by the demodulation unit 111 in synchronization with the sampling clock, a frame synchronization detection unit 123 for outputting a frame synchronization detection signal when bit streams of the frame code register 121 and the frame synchronization shift register 122 are identical, and a reception buffer 124 for storing the reception data in synchronization with the sampling clock when a frame synchronization is detected by the frame synchronization detection unit 123.
The microcomputer unit 1300 includes a RAM 131 for storing reception data, a controller 132 for decoding an original message from the reception data stored in the RAM 131, and a transmission unit 133 for transmitting the reception data stored in the reception buffer 124 to the RAM 131 by the number of times designated by the controller 132, and outputting a transmission completion signal to the controller 132 when the transmission of the reception data is completed.
Hereinafter, a reception operation of the conventional example will be described with reference to a time chart shown in FIG. 16. Also, a communications frame exchanged in the conventional example includes a synchronization bit stream (preamble) for allowing bit synchronization, a frame synchronization bit stream (unique word) for allowing frame synchronization, data including a communications message, a check code (e.g., a CRC) for an error detection, and the like.
First, the microcomputer unit 1300 awaits in a sleep mode until a frame synchronization detection signal is outputted from the frame synchronization detection unit 123 of the interface unit 1200. And, when the RF unit 1100 receives a radio signal and a frame synchronization detection signal is outputted from the frame synchronization detection unit 123 of the interface unit 1200, the microcomputer unit 1300 starts a rising edge interrupt process in synchronization with rising of the frame synchronization detection signal.
When the microcomputer unit 1300 starts the rising edge interrupt process, the controller 132 thereof instructs the interface unit 1200 to output the reception data stored in the reception buffer 124. In the microcomputer unit 1300, the reception data outputted from the reception buffer 124 is transmitted to the RAM 131 by the transmission unit 133, and the controller 132 decodes it into the original message.
Further, when a bit stream of a prescribed length is received from the reception buffer 132, the controller 132 outputs a reset signal to the RF unit 1100 and the interface unit 1200. When the RF unit 1100 and the interface unit 1200 receive the reset signal from the controller 132, the sampling clock generating unit 112 and the frame synchronization detection unit 123 are reset.
In the above conventional example, normally, only the RF unit 1100 and the interface unit 1200 operate and the microcomputer unit 1300 is in a sleep mode, thereby reducing power consumption. Further, since a processing load of the microcomputer unit 1300 is reduced during standby, an inexpensive (low performance) microcomputer may be used.
Herein, it happens that the demodulation unit 111 of the RF unit 1100 outputs a signal such as a random bit stream due to the influence of thermal noise or radio wave noise even while the antenna 1000 is not receiving a radio signal. Further, the likelihood is that the random bit stream includes the same bit stream as the bit stream (unique word) of the frame synchronization part. Accordingly, the frame synchronization detection unit 123 may erroneously detect a frame synchronization part and output a frame synchronization detection signal.
Even in this case, the microcomputer unit 1300 starts a rising edge interrupt process in synchronization with rising of the frame synchronization detection signal and the controller 132 instructs the interface unit 1200 to output the reception data stored in the reception buffer 124. Further, in the microcomputer unit 1300, the reception data outputted from the reception buffer 124 is transmitted to the RAM 131 by the transmission unit 133, and the controller 132 decodes it into the original message (see FIG. 17).
In the meantime, the sampling clocking generating unit 112 of the RF unit 1100 continuously monitors a bit stream of the demodulation signal demodulated by the demodulation unit 111. Since a bit width (pulse width) of the random bit stream is not uniform, the sampling clock generating unit 112 determines soon that there is out of synchronization and stops outputting of the sampling clock. When outputting of the sampling clock is stopped, the frame synchronization detection unit 123 also stops outputting of the frame synchronization detection signal.
Further, when the frame synchronization detection signal falls before a bit stream having a prescribed length is received from the reception buffer 124, the microcomputer unit 1300 starts a falling edge interrupt process in which the data (bit stream) received from the reception buffer 124 is discarded and a reset signal is outputted to the RF unit 1100 and the interface unit 1200, and then turns to be in standby status (see FIG. 17).
[Patent Document 1] Japanese Patent Application Publication No. 2010-28331
[Patent Document 2] Japanese Patent Application Publication No. 2008-176515
[Patent Document 3] Japanese Patent Application Publication No. 2006-239731
By the way, among the two types of local oscillators described above, the local oscillator using a frequency multiplier circuit advantageously consumes less power in comparison to the local oscillator using a PLL circuit. On the contrary, the latter local oscillator has a wider variable range of frequency than that of the former local oscillator. In many cases, general wireless transceivers employ the local oscillator using a PLL circuit in consideration of the fact that the variable range of frequency is wide. However, the use of the PLL circuit increases power consumption in comparison to the case of using a frequency multiplier circuit. Especially, in case where a wireless transceiver is mounted in the device which uses a battery as a power source, it is preferred that the former local oscillator (i.e., the local oscillator using a frequency multiplier circuit) which consumes less power is employed, to thereby lengthening a life span of the battery.
In the fire warning system of Patent Document 2, the operation controller is intermittently operated in order to reduce power consumption, and the operation controller started up by a timer checks a state of the reception signal received by the wireless reception/transmission unit, i.e., determines whether or not a radio wave can be received based on a measurement result of a reception signal strength.
Here, when it is determined that the radio wave cannot be received, the operation controller stops a transmission/reception operation of the wireless transmission/reception unit and then transits its operation state to a sleep state. However, the operation controller keeps operating while the wireless transmission/reception unit is measuring the reception signal strength, which results in unnecessary power consumption and reducing a life span of the battery as much.
Further, in Patent Document 3, if a regular radio signal is received immediately after an erroneous synchronization occurs due to thermal noise or radio wave noise, there is a possibility that the regular radio signal is not received normally. The case in which such a phenomenon occurs will be described with reference to the time chart shown in FIG. 18. In FIG. 18, N is a random value obtained by demodulating thermal noise, P is a preamble, U is a unique word, and 1,2,3 . . . are data.
It is assumed that a frame synchronization detection signal rises due to an error detection at the time of t1, the microcomputer unit 1300 starts a rising edge interrupt process, and a regular radio signal is received in succession. Since a pulse width of a bit stream of a demodulation signal change at the time of t9 when the regular radio signal is inputted, the sampling clock generating unit 112 often determines that it is a synchronization loss.
Here, there may occur a case where the frame synchronization detection signal falls due to a synchronization loss at the time t2 before the controller 132 outputs, at the time t3, a control signal for instructing the interface unit 120 to output the reception data stored in the reception buffer 124. In this case, since the control signal is outputted from the microcomputer unit 1300 although the frame synchronization detection signal has fallen due to the synchronization loss, reception data is outputted from the reception buffer 124 to the microcomputer unit 1300 in synchronization with falling of the control signal at the time t4.
Further, the microcomputer unit 1300 detects falling of the frame synchronization detection signal since the rising edge interrupt processing is finished, starts a falling edge interrupt process, and discards accumulated reception data (t=t5).
Meanwhile, after the time t2 at which the synchronization loss is determined, the frame synchronization detection unit 123 detects a unique word from the demodulation signal of the regular radio signal and accordingly the frame synchronization detection signal rises (t=t5), while the microcomputer unit 1300 is executing the falling edge interrupt process at the time t5. When the falling edge interrupt process is terminated (t=t6), the microcomputer unit 1300 detects rising of the frame synchronization detection signal and starts a rising edge interrupt process, and the controller 132 outputs a control signal for instructing the interface unit 1200 to output the reception data stored in the reception buffer 124 (time t=t8). Accordingly, the reception data starts to be accumulated at the time t8. However, the reception data is continuously outputted from the time t4 of the output instruction caused by the erroneous detection.
That is, the reception data of the regular radio signal had already been started to be outputted from the buffer 124 at the time t7 when the control signal is output from the microcomputer unit 1300. Therefore, although the microcomputer unit 1300 starts to receive data at the time t8 when the control signal falls, data corresponding to 3 bits already outputted cannot be received. Further, the microcomputer unit 1300 instructs the interface unit 1200 to discard the reception data at the timing when the reception data is deleted. Accordingly, even when a control signal for instructing to accumulate reception data is outputted after a next frame synchronization detection, the reception data may not be accumulated in the reception buffer 124 because it is after the frame synchronization detection signal has been risen (the time t=t5).